Compliant micro device transfer head with integrated electrode leads

ABSTRACT

A compliant micro device transfer head and head array are disclosed. In an embodiment a micro device transfer head includes a spring arm having integrated electrode leads that is deflectable into a space between a base substrate and the spring arm.

BACKGROUND

1. Field

The present invention relates to micro devices. More particularly,embodiments of the present invention relate to a compliant micro devicetransfer head and transfer head array and a method for forming acompliant micro device transfer head and transfer head array.

2. Background Information

Integration and packaging issues are one of the main obstacles for thecommercialization of micro devices such as integration of radiofrequency (RF) microelectromechanical systems (MEMS) microswitches,light-emitting diode (LED) integration onto image display systems, andMEMS or quartz-based oscillators.

Traditional technologies for transferring of devices include transfer bywafer bonding from a transfer wafer to a receiving wafer. One suchimplementation is “direct printing” involving one bonding step of anarray of devices from a transfer wafer to a receiving wafer, followed byremoval of the transfer wafer. Another such implementation is “transferprinting” involving two bonding/debonding steps. In transfer printing atransfer wafer may pick up an array of devices from a donor wafer, andthen bond the array of devices to a receiving wafer, followed by removalof the transfer wafer.

Some printing process variations have been developed where a device canbe selectively bonded and debonded during the transfer process. Still,in both traditional and variations of the direct printing and transferprinting technologies, the transfer wafer must be debonded from a deviceafter bonding the device to the receiving wafer. In addition, the entiretransfer wafer with the array of devices is involved in the transferprocess.

SUMMARY OF THE INVENTION

A compliant micro device transfer head and head array and method offorming a compliant micro device transfer head and head array aredisclosed. In an embodiment, the micro device transfer head includes abase substrate, a spring anchor coupled to the base substrate, a springarm coupled to the spring anchor, and a dielectric layer. In anembodiment, the spring arm includes a mesa structure suspended over thebase substrate, a top electrode extending laterally from the springanchor and over the mesa structure, and a bottom electrode extendinglaterally from the spring anchor underneath the mesa structure andwrapping around and over the mesa structure. In an embodiment, thedielectric layer covers the top electrode and the bottom electrode overthe mesa structure.

The spring arm may further include a template filling an internal volumedefined by the top electrode and the bottom electrode, where thetemplate includes the mesa structure. The template may be formed from aninsulating material, such as silicon dioxide, nitride, or air. The topelectrode and the bottom electrode may be metal. In an embodiment, thedielectric layer covers a top surface of the top electrode. The microdevice transfer head may further include a strain sensor formed over thedielectric layer at an interface of the spring anchor and the springarm. The mesa structure may be suspended over an underlying space. In anembodiment, the spring arm is coupled to the base substrate via adielectric layer. The underlying space may be a cavity in the dielectriclayer. The micro device transfer head may additionally include a sensorelectrode on a bottom surface of the underlying space, such that thesensor electrode is vertically aligned with the mesa structure. Themicro device transfer head may further include routing in the basesubstrate, wherein the routing is electrically connected to the topelectrode and bottom electrode.

In an embodiment, a method for forming a transfer head includes forminga dielectric layer over a substrate, forming a first electrode layerover the dielectric layer, forming a template on the first electrodelayer, where the template includes a mesa structure, forming a secondelectrode layer over the template, patterning the second electrode layerand first electrode layer to form a bottom electrode and a topelectrode, and removing a portion of the dielectric layer fromunderneath the bottom electrode to form a spring arm coupled to thesubstrate by a spring anchor. The spring arm may include the mesastructure, such that the mesa structure is suspended over the substrate,the top electrode extending laterally from the spring anchor and overthe mesa structure, and the bottom electrode extending laterally fromthe spring anchor underneath the mesa structure and wrapping around andover the mesa structure.

In an embodiment, patterning the second electrode layer and firstelectrode layer includes forming a first negative pattern on thesubstrate defining a first portion of a bottom electrode, sputtering thefirst electrode layer over the substrate and the first negative pattern,and removing the first negative pattern along with portions of the firstelectrode layer formed on the first negative pattern. In an embodiment,patterning the second electrode layer and first electrode layerincludes, forming a second negative pattern on the template, wherein thesecond negative pattern defines the top electrode and a second portionof the bottom electrode, forming the second electrode layer over thetemplate and the second negative pattern, and removing the secondnegative pattern along with portions of the second electrode layerformed over the second negative pattern. In another embodiment,patterning the second electrode layer and first electrode layer includesforming a pattern to define a first portion of the bottom electrode inthe first electrode layer and patterning to remove exposed portions ofthe first electrode layer to form the first portion of the bottomelectrode. In another embodiment, patterning the second electrode layerand first electrode layer includes forming a pattern to define the topelectrode and a second portion of the bottom electrode in the secondelectrode layer and patterning to remove exposed portions of the secondelectrode layer to form the second portion of the bottom electrode. Thetemplate may be formed from an insulating material or from photoresist.The method may further include removing the template. In an embodiment,the method further includes forming a dielectric layer covering the topelectrode and the bottom electrode over the mesa structure.

In an embodiment of the invention, a micro device transfer head arrayincludes a base substrate and an array of transfer heads. In anembodiment, each transfer head includes a spring anchor coupled to thebase substrate, a spring arm coupled to the spring anchor, and adielectric layer covering the top electrode and the bottom electrodeover the mesa structure. In an embodiment, the spring arm includes amesa structure suspended over the base substrate, a top electrodeextending laterally from the spring anchor and over the mesa structure,and a bottom electrode extending laterally from the spring anchorunderneath the mesa structure and wrapping around and over the mesastructure. The micro device transfer head array may further includerouting in the base substrate, wherein the routing is electricallyconnected to the top electrode and bottom electrode. The micro devicetransfer head array may further include a conductive ground plane formedover the dielectric layer and surrounding each of the transfer heads.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view illustration of a micro devicetransfer head in accordance with an embodiment of the invention.

FIG. 2 is an isometric illustration of a micro device transfer head inaccordance with an embodiment of the invention.

FIGS. 3A-3B are isometric illustrations of a micro device transfer headarray in accordance with an embodiment of the invention.

FIGS. 4A-4B are cross-sectional side view illustrations of an array ofmicro device transfer heads picking up an array of micro LED devices inaccordance with an embodiment of the invention.

FIGS. 5A-5B are cross-sectional side view illustrations of sensorcomponents of a micro device transfer head in accordance with anembodiment of the invention.

FIGS. 6A-6I are cross-sectional side view illustrations of a method forforming a micro device transfer head in accordance with an embodiment ofthe invention.

FIGS. 7A-7G are cross-sectional side view illustrations of sensorcomponents of a cantilever micro device transfer head in accordance withan embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention describe a compliant micro devicetransfer head and head array, and a method of formation. In variousembodiments, description is made with reference to figures. However,certain embodiments may be practiced without one or more of thesespecific details, or in combination with other known methods andconfigurations. In the following description, numerous specific detailsare set forth, such as specific configurations, dimensions andprocesses, etc., in order to provide a thorough understanding of thepresent invention. In other instances, well-known semiconductorprocesses and manufacturing techniques have not been described inparticular detail in order to not unnecessarily obscure the presentinvention. Reference throughout this specification to “one embodiment,”“an embodiment” or the like means that a particular feature, structure,configuration, or characteristic described in connection with theembodiment is included in at least one embodiment of the invention.Thus, the appearances of the phrase “in one embodiment,” “an embodiment”or the like in various places throughout this specification are notnecessarily referring to the same embodiment of the invention.Furthermore, the particular features, structures, configurations, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

The terms “over”, “to”, “between” and “on” as used herein may refer to arelative position of one layer with respect to other layers. One layer“over” or “on” another layer or bonded “to” another layer may bedirectly in contact with the other layer or may have one or moreintervening layers. One layer “between” layers may be directly incontact with the layers or may have one or more intervening layers.

The terms “micro” device or “micro” LED structure as used herein mayrefer to the descriptive size of certain devices or structures inaccordance with embodiments of the invention. As used herein, the terms“micro” devices or structures are meant to refer to the scale of 1 to100 μm. However, it is to be appreciated that embodiments of the presentinvention are not necessarily so limited, and that certain aspects ofthe embodiments may be applicable to larger, and possibly smaller sizescales. In an embodiment, a single micro device in an array of microdevices, and a single electrostatic transfer head in an array ofelectrostatic transfer heads both have a maximum dimension, for examplelength or width, of 1 to 100 μm. In an embodiment, the top surface ofthe transfer head has a maximum dimension of 3 to 10 μm. In anembodiment a pitch of an array of micro devices, and a correspondingarray of electrostatic transfer heads is (1 to 100 μm) by (1 to 100 μm),for example a 10 μm by 10 μm pitch or 5 μm by 5 μm pitch.

In one aspect, embodiments of the invention describe an array ofcompliant transfer heads that may be used for mass transfer of an arrayof pre-fabricated micro devices. For example, the pre-fabricated microdevices may have a specific functionality such as, but not limited to,an LED for light-emission, silicon IC for logic and memory, and galliumarsenide (GaAs) circuits for radio frequency (RF) communications. Insome embodiments, arrays of micro LED devices, which are poised for pickup, are described as having a 10 μm by 10 μm pitch, or 5 μm by 5 μmpitch. At these densities a 6 inch substrate, for example, canaccommodate approximately 165 million micro LED devices with a 10 μm by10 μm pitch, or approximately 660 million micro LED devices with a 5 μmby 5 μm pitch. A transfer tool including an array of transfer headsmatching an integer multiple of the pitch of the corresponding array ofmicro LED devices can be used to pick up and transfer the array of microLED devices to a receiving substrate. For example, the micro device maybe contacted with the transfer head and a voltage applied to a pair ofelectrodes in the transfer head to create a grip pressure on the microdevice. The transfer head picks up the micro device and then releasesthe micro device onto a receiving substrate. In this manner, it ispossible to integrate and assemble micro LED devices intoheterogeneously integrated systems, including substrates of any sizeranging from micro displays to large area displays, and at high transferrates. For example, a 1 cm by 1 cm array of micro device transfer headscan pick up and transfer more than 100,000 micro devices, with largerarrays of micro device transfer heads being capable of transferring moremicro devices. Each compliant transfer head in the array of complianttransfer heads may also be independently controllable, which enablesselective pick up and release of the micro devices.

In another aspect, the spring arm can compensate for variations inheight of the micro devices or for particulate contamination on top of amicro device. For example, one end of the spring arm is anchored to thesubstrate by a spring anchor, while the opposing end of the spring armis suspended over the substrate. A spring arm associated with a talleror contaminated micro device may deflect towards the substrate more thana spring member associated with a shorter micro device in an array ofmicro devices, so that each compliant transfer head may make contactwith each micro device, ensuring that each intended micro device ispicked up.

In another aspect, the electrode components of the transfer head are anintegral part of the compliant transfer head structure. For example, atop and bottom electrode function both structurally, to enable thecompliant spring effect of the spring arm portion of the transfer head,and electrically, to generate the electrostatic grip pressure that picksup the micro device. In an embodiment, the electrodes solely determinethe structural and deflection characteristics of the spring arm. Inanother embodiment, the volume between the top electrode and bottomelectrode is filled with a gap material that also contributes to thestructural and deflection characteristics of the spring arm.

In an embodiment, the spring arm includes two electrodes: a topelectrode extending from the spring anchor and over a mesa structure,and a bottom electrode extending from the spring anchor, underneath themesa structure, and wrapping around and over the mesa structure. In thismanner, a bipolar electrode is formed on the top surface of the mesastructure. The top surface of the mesa structure is sized to correspondto the area of the top surface of the micro devices to be picked up. Adielectric layer is formed over the top and bottom electrodes over themesa structure to provide a suitable surface for contacting a microdevice with the desired grip pressure.

Each transfer head may additionally include a sensor, which can monitoran amount of deflection of the spring arm when the transfer head isbrought into contact with a micro device, enabling detection of whethercontact has been made with a micro device, whether a micro device isirregularly shaped or contaminated, or whether a micro device isattached to the transfer head and has successfully been picked up.

In another aspect, a method for forming the transfer head includesforming electrodes over a template comprising a mesa structure. In anembodiment, the template is retained as part of the transfer head. Inanother embodiment, the template is removed, resulting in a hollowtransfer head defined by the top and bottom electrodes. The electrodescan be patterned, for example, by a negative liftoff method, where themetal electrode material is formed over a photoresist pattern, which isthen removed to leave metal where no photoresist was formed. Theelectrodes can alternatively be patterned formed by deposition andpatterning, for example, by conventional lithography.

Referring now to FIG. 1, a cross-sectional side view of a complianttransfer head 100 is illustrated in accordance with an embodiment of theinvention. The transfer head 100 includes a spring arm 110 coupled todielectric layer 101 on base substrate 102 via spring anchor 120. Springarm 110 includes a top electrode 106, a bottom electrode 108, and a mesastructure 104 having a top surface 103 and tapered sidewalls 105A/105B.Top electrode 106 extends laterally from the spring anchor 120 and overthe top surface 103 of a mesa structure 104. Bottom electrode 108extends laterally from the spring anchor 120, underneath the mesastructure 104 and wraps around and over the top surface 103 of mesastructure 104. A dielectric layer 114 covers the top electrode 106 andbottom electrode 108 on the top surface 103 of mesa structure 104.

Base substrate 102 may be formed from a variety of materials such assilicon, ceramics and polymers that are capable of providing structuralsupport. Base substrate 102 may have a conductivity between 10³ and 10¹⁸ohm-cm. In an embodiment, base substrate 102 has a dielectric layer 101disposed thereon. In an embodiment, dielectric layer 101 electricallyisolates bottom electrode 108 from a semiconductor substrate 102.Dielectric layer 101 may be any appropriate dielectric material, forexample silicon dioxide or silicon nitride. In an embodiment, thethickness of dielectric layer 101 is selected to correspond to thedesired depth of space 112. For example, the thickness of dielectriclayer 101 may be from 0.5 μm to 2 μm. In an embodiment, space 112 is acavity in dielectric layer 101. In another embodiment, space 112 is acavity in substrate 102.

In an embodiment, base substrate 102 and dielectric layer 101 includerouting, such as through vias 118A, 118B, to connect the micro devicetransfer head 100 to the working electronics of an electrostatic gripperassembly. In an embodiment, a through via 118A connects to top electrode106 and a through via 118B connects bottom electrode 108, as shown inFIG. 1. It is to be understood that base substrate 102 and dielectriclayer 101 may include additional routing beyond what is illustrated inFIG. 1.

In an embodiment, top electrode 106 and bottom electrode 108 wrap arounda template comprising gap material 116 and mesa structure 104. In anembodiment, gap material 116 extends from spring anchor 120, separatingbottom electrode 108 from top electrode 106. In an embodiment, the gapmaterial 116 possesses a dielectric strength greater than the appliedelectric field so as to avoid shorting between top electrode 106 andbottom electrode 108 during operation of the transfer head. As such, gapmaterial 116 may be formed from a variety of materials includingphotoresist, polyimide, amorphous silicon, polysilicon, or dielectricmaterials, for example inorganic dielectrics such as silicon dioxide andsilicon nitride, or organic dielectrics such as benzocyclobutene (BCB).In an embodiment, gap material 116 is filled with air. The thickness ofgap material 116 may depend on a number of factors, such as thedimensions of the electrodes, the amount of deflection of the springarm, or the voltage applied to the electrodes during a pickup operation.In an embodiment, gap material 116 is from 2 μm to 5 μm thick. Inaccordance with the illustrated embodiment, gap material 116 covers thesurface of dielectric layer 101 that surrounds space 112, such as thatshown adjacent to the suspended end of spring arm 110.

Mesa structure 104 is suspended above space 112, opposite the springanchor 120 on spring arm 110, according to an embodiment of theinvention. In an embodiment, mesa structure 104 protrudes away from basesubstrate 102. Mesa structure 104 has top surface 103, which may beplanar, and tapered sidewalls 105A/105B. In an embodiment, sidewalls105A/105B may be tapered up to 10 degrees, for example. Tapering thesidewalls 105A/105B may be beneficial in forming the electrodes 106/108as described further below. Mesa structure 104 may be formed from thesame or different material than gap material 116. In an embodiment, mesastructure 104 is a patterned oxide layer, such as silicon dioxide. Inanother embodiment, mesa structure 104 is filled with air.

The dimensions of mesa structure 104 may depend upon the specificdimensions of the micro devices to be picked up, as well as thethickness of any layers formed over the mesa structure. The specificdimensions of mesa structure 104 will be discussed below with referenceto FIG. 2. In an embodiment, the height, width, and planarity of anarray of mesa structures on a base substrate are uniform across the basesubstrate so that each micro device transfer head is capable of makingcontact with each corresponding micro device during the pick upoperation. In an embodiment, the surface area of the top surface 115 ofeach micro device transfer head 100 is slightly larger, approximatelythe same, or less than the surface area of the top surface of the eachmicro device in the corresponding micro device array so that a transferhead does not inadvertently make contact with a micro device adjacent tothe intended corresponding micro device during the pick up operation.Since additional layers may be formed over the mesa structure 104 (e.g.electrodes 106/108, and dielectric layer 114) the surface area of themesa structure may account for the thickness of the overlying layers sothat the surface area of the top surface 115 of each micro devicetransfer head is slightly larger, approximately the same, or less thanthe surface area of the top surface of the each micro device in thecorresponding micro device array.

In an embodiment, top electrode 106 covers a portion of top surface 103of mesa structure 104, runs down sidewall 105A of the mesa structure,and along the top surface of gap material 116 to spring anchor 120. Topelectrode 106 may then connect to conductive routing, such as a throughvia 118A, in dielectric layer 101 and substrate 102. In an embodiment,bottom electrode 108 covers a portion of top surface 103 of mesastructure 104 and wraps around sidewall 105B, running underneath mesastructure 104 and gap material 116 to spring anchor 120. Bottomelectrode 108 may also connect to conductive routing in dielectric layer101 and substrate 102, for example a through via 118B. In an embodiment,the portions of top electrode 106 and bottom electrode 108 formed overthe top surface 103 of mesa structure 104 are separated by a gap 107. Inaccordance with an embodiment, electrodes 106/108 cover the maximumamount of surface area of the top surface 103 of the mesa structure 104as possible while remaining within patterning tolerances. Minimizing theamount of free space increases the capacitance and resultant grippressure that can be achieved by the micro device transfer head.

The material from which electrodes 106/108 are formed is selected tohave a suitable elastic modulus to meet the structural and deflectionrequirements of the spring arm and also a suitable electricalconductivity to meet the electrical requirements of the electrostaticgrip function of the transfer head, according to an embodiment of theinvention. A variety of conductive materials including metals, metalalloys, refractory metals, and refractory metal alloys may be employedto form top electrode 106 and bottom electrode 108. In an embodiment,electrodes 106/108 include a high melting temperature metal such asplatinum or a refractory metal or refractory metal alloy. For example,an electrode may include platinum, titanium, vanadium, chromium,zirconium, niobium, molybdenum, ruthenium, rhodium, hafnium, tantalum,tungsten, rhenium, osmium, iridium and alloys thereof. Refractory metalsand refractory metal alloys generally exhibit higher resistance to heatand wear than other metals. Top electrode 106 and bottom electrode 108may be from 100 to 1000 nm thick. In an embodiment, electrodes 106/108are each an approximately 500 Å (0.05 μm) thick layer of titaniumtungsten (TiW) refractory metal alloy.

In an embodiment, dielectric layer 114 is formed on the surfaces ofspring arm 110 and underlying space 112. In an embodiment, dielectriclayer 114 covers the exposed surfaces of the top electrode 106 andbottom electrode 108 forming the spring arm 110. The dielectric layer114 may also cover other exposed layers on transfer head 100, dielectriclayer 101, substrate 102, and gap material 116. In an embodiment, thedielectric layer 114 has a suitable thickness and dielectric constantfor achieving the required grip pressure of the micro device transferhead 100, and sufficient dielectric strength to not break down at theoperating voltage. The dielectric layer 114 may be a single layer ormultiple layers. In an embodiment, the dielectric layer 114 is 0.5 μm to2.0 μm thick, though the thickness may be more or less depending uponthe specific topography of the transfer head 100 and underlying mesastructure 104. Suitable dielectric materials may include, but are notlimited to, aluminum oxide (Al₂O₃) tantalum oxide (Ta₂O₅), halfnium(HfO₂), and silicon dioxide (SiO₂). In accordance with embodiments ofthe invention, the dielectric layer 114 possesses a dielectric strengthgreater than the applied electric field so as to avoid shorting of thetransfer head during operation. The dielectric layer 113 can bedeposited by a variety of suitable techniques such as chemical vapordeposition (CVD), atomic layer deposition (ALD) and physical vapordeposition (PVD) such as sputtering. The dielectric layer 114 mayadditionally be annealed following deposition. In one embodiment, thedielectric layer 114 possesses a dielectric strength of at least 400V/μm. Such a high dielectric strength can allow for the use of a thinnerdielectric layer. Techniques such as ALD can be utilized to deposituniform, conformal, dense, and/or pin-hole free dielectric layers withgood dielectric strength. Multiple layers can also be utilized toachieve such a pin-hole free dielectric layer. Multiple layers ofdifferent dielectric materials may also be utilized to form dielectriclayer 114. In an embodiment, the underlying electrodes 106/108 includeplatinum or a refractory metal or refractory metal alloy possessing amelting temperature above the deposition temperature of the dielectriclayer material(s) so as to not be a limiting factor in selecting thedeposition temperature of the dielectric layer 114.

The materials and dimensions of the components of transfer head 100 areselected to enable deflection of spring arm 110 into space 112 over theworking temperature range of the micro device transfer process. In anembodiment, the components and dimensions of spring arm 110 are selectedto enable deflection of approximately 0.5 μm into space 112 when the topsurface of transfer head 100 is subjected to up to 10 atm of pressure atoperating temperatures up to 350° C.

Referring now to FIG. 2, an isometric view of a compliant transfer head100 is illustrated in accordance with an embodiment of the invention.Certain features and components of the transfer head 100 that areillustrated in FIG. 1 are omitted in FIG. 2 in order to more clearlyillustrate other features. For example, the dielectric layer coveringelectrodes 106/108 has been omitted.

Spring arm 110 has a thickness T_(SA), width W_(SA), and length L_(SA),according to an embodiment of the invention. The thickness T_(SA) ofspring arm 110 depends on the elastic moduli and dimensions of thematerials and components from which it is formed. In an embodiment,spring arm 110 has a thickness T_(SA) of up to 8 μm. In an embodiment,the width W_(SA) of spring arm 110 is sufficient to accommodateadditional spring arm and transfer head elements, such as electrodes106/108 and mesa structure 104. For example, where a micro device is 3μm to 5 μm wide, the width W_(SA) of the spring arm may also be 3 μm to5 μm, and where a micro device is 8 μm to 10 μm wide, the width W_(SA)of the spring arm may also be 8 μm to 10 μm. The length L_(SA) of springarm 110 is long enough to enable spring arm 110 to deflect into space112, but less than the pitch of the transfer heads 100 in the transferhead array. In an embodiment, the length L_(SA) of spring arm 110 is 10×longer than the width W_(SA). In an embodiment, the length L_(SA) ofspring arm 124 may be from 10 μm to 100 μm.

Gap material 116 extends laterally from the spring anchor 120 betweenbottom electrode 108 and top electrode 106, according to an embodimentof the invention. In an embodiment, the thickness T_(G) of gap material116 depends on a number of factors, such as the dimensions of theelectrodes, the desired amount of deflection of the spring arm, thevoltage applied to the electrodes during a pickup operation. In anembodiment, the thickness T_(G) of gap material 116 is from 2 μm to 5μm.

Mesa structure 104 generates a profile that protrudes away from thesurface of gap material 116 so as to provide a localized contact pointto pick up a specific micro device during a pick up operation, accordingto an embodiment of the invention. The height of mesa structure 104elevates the surface of the transfer head that will contact the microdevice to be picked up, enabling targeted pickup of a correspondingmicro device in a micro device array. In an embodiment, mesa structure104 has a height H_(M) of approximately 1 μm to 5 μm, or morespecifically approximately 2 μm. The surface area dimensions(L_(M)×W_(M)) of mesa structure 104 may be sized to correspond to thearea of the top surface of the micro device to be picked up. The surfacearea of the mesa structure may account for the thickness of electrodes106/108 and the dielectric layer (not shown) so that the surface area ofthe top surface of each micro device transfer head is slightly larger,approximately the same, or less than the surface area of the top surfaceof the each micro device in the corresponding micro device array. In anexemplary embodiment, the length L_(M) and width W_(M) of mesa structure104 are each from 2 μm to 7 μm in order to achieve a top surface of thetransfer head 100 having length and width dimensions that are each from3 μm to 10 μm.

The electrodes 106/108 may cover the maximum amount of the surface areaof the top surface 103 of the mesa structure 104 as possible while stillpreserving gap 107 between top electrode 106 and bottom electrode 108.The minimum amount of separation distance may be balanced byconsiderations for maximizing surface area, while avoiding electricalbreakdown between the electrodes. For example, gap 107 may be 0.5 μm orless, and the minimum separation distance may be limited by thethickness of the electrodes. It is to be appreciated that the mesa arraymay have a variety of different pitches, and that embodiments of theinvention are not limited to the exemplary 7 μm×7 μm top surface 103 ofthe mesa structure 104 in a 10 μm pitch.

FIGS. 3A-3B are isometric view illustrations of an array of 300A/300B ofmicro device transfer heads 100 with a compliant bipolar electrode aspreviously described with regard to FIGS. 1-2. For purposes of clarity,certain elements of the transfer head have not been illustrated, forexample, the dielectric layer covering the top surface of each mesastructure. In FIG. 3A, each transfer head 100 in transfer head array300A is deflectable into an underlying space 112, according to anembodiment of the invention. FIG. 3B illustrates an embodiment of atransfer head array 300B where each transfer head 100 is deflectableinto underlying trench 332 that extends underneath multiple transferheads 100.

Each top electrode 106 of a transfer head 100 is coupled to a bus line330, according to an embodiment of the invention. Each bottom electrode108 may also be coupled to a separate bus line (not illustrated). In anembodiment, bus line 330 is connected to additional routing within thedielectric layer 101 and substrate 102. It is to be appreciated thatadditional conductive routing or different routing configurations may beused to control the transfer heads 100. In an embodiment, a conductiveground plane (not shown) is formed over the dielectric layer 101,surrounding the array of transfer heads 100 to assist in the preventionof arcing between transfer heads, particularly during the application ofhigh operating voltages.

FIGS. 4A-4B are cross-sectional side view illustrations of an array ofmicro device transfer heads, taken along the line A-A′ in FIG. 3, shownpicking up an array of micro devices in accordance with an embodiment ofthe invention. In FIG. 4A, an array of transfer heads 100 is positionedabove an array of micro LED devices 400 on carrier substrate 401,according to an embodiment. In the particular embodiment illustrated,the pitch P_(TH) of the array of transfer heads 100 is an integermultiple of the pitch P_(MD) of the micro LED devices 400, with thepitch P_(TH) of the array of transfer heads being the sum of the spacingS_(TH) between the top surfaces 115 of adjacent transfer heads 100 andthe width W_(TH) of the top surface 115 of a transfer head 100. Inanother embodiment, the pitch P_(TH) of the transfer heads is equal tothe pitch P_(MD) of the micro LED devices.

In one embodiment, the array of micro LED devices 400 have a pitchP_(MD) of 10 μm, with each micro LED device having a spacing S_(MD) of 2μm and a maximum width W_(MD) of 8 μm. In an exemplary embodiment, thetop surface of the each micro LED device 400 has a width W_(MD) ofapproximately 8 μm. In such an exemplary embodiment, the width W_(TH) ofthe top surface 115 of a corresponding transfer head 100 isapproximately 10 μm or smaller so as to avoid making inadvertent contactwith an adjacent micro LED device. In another embodiment, the array ofmicro LED devices 400 may have a pitch P_(MD) of 5 μm, with each microLED device having a spacing S_(MD) of 2 μm and a maximum width W_(MD) of3 μm. In an exemplary embodiment, the top surface of the each micro LEDdevice 400 has a W_(MD) width of approximately 3 μm. In such anexemplary embodiment, the width W_(TH) of the top surface 115 of acorresponding transfer head 100 is approximately 3 μm or smaller so asto avoid making inadvertent contact with an adjacent micro LED device400. However, embodiments of the invention are not limited to thesespecific dimensions, and may be any suitable dimension.

FIG. 4B is a side view illustration of the array of micro devicetransfer heads picking up a portion of an array of micro LED devices 400in accordance with an embodiment of the invention. In a particularembodiment illustrated in FIG. 4B, every transfer head 100 in the arrayis activated to pick up a micro LED device 400. In another embodiment,transfer heads 100 may be selectively activated, such that a subset ofthe array of transfer heads 100 picks up a micro LED device 400.

The portion of the array of micro devices may then be released onto atleast one receiving substrate to complete the transfer process. Thus,the array of micro LED devices can all be released onto a singlereceiving substrate, or selectively released onto multiple substrates.For example, the receiving substrate may be, but is not limited to, adisplay substrate, a lighting substrate, a substrate with functionaldevices such as transistors or ICs, or a substrate with metalredistribution lines.

FIGS. 5A-5B each illustrate an embodiment of a micro device transferhead incorporating a sensor. Sensors can serve a variety of purposesduring operation of the transfer head. For example, where a sensor isused to measure an amount of deflection of the transfer head, thisinformation can be used to determine if (1) contact has been made with amicro device to be picked up, (2) contamination is present on the microdevice, or alternatively the micro device has been damaged or deformed,or (3) whether a micro device has been picked up.

FIG. 5A illustrates a cross sectional side view of a transfer headcomprising a strain sensor 540, according to an embodiment of theinvention. In an embodiment, strain sensor 540 is a strain gauge capableof measuring the amount of deflection of spring arm 110 into space 112.When the top surface 115 of transfer head 100 contacts the surface of amicro device during a pick up operation, it may deflect some amount inresponse to the contact pressure. By measuring the amount of deflectionof a spring arm 110 and comparing it to the amount of deflection knownto indicate optimum contact with a micro device surface, strain sensor540 can indicate whether the top surface 115 of transfer head 100 hascontacted the top surface of a micro device in an array and as such isready to execute a pick up operation. Detection of too little deflectionmay indicate that a micro device is absent from that position in thearray, while detection of too much deflection may indicate separation orincomplete contact between the surface of the micro device and thesurface of the transfer head due to either the presence of contaminationparticles or an otherwise damaged or deformed micro device. In bothcases, a voltage may not be applied to the transfer head so as not toattempt to pick up the absent or damaged micro device. In the case wherecontamination is detected, a cleaning operation may be applied to thetransfer head, micro device, or their respective array prior toreattempting the pick up operation.

In another embodiment, strain sensor 540 is capable of measuring aresonant frequency of spring arm 100. For example, a resonant frequencyof spring arm 110 may be measured upon contacting the transfer head on abare substrate and lifting off of the bare substrate without a microdevice. A different resonant frequency of spring arm 110 may be measuredupon contacting the transfer head to a micro device and picking up themicro device. As such, the strain sensor 540 is capable of detectingwhether a micro device has been picked up, due to differences in theresonant frequency when a micro device is or is not present.

In an embodiment, strain sensor 540 is formed on dielectric layer 114over the interface of spring anchor 120 and spring arm 110. In anembodiment, dielectric layer 114 electrically isolates sensor 540 fromtop electrode 106. When spring arm 110 deflects into space 112, thestrain along spring arm 110 is not uniform; spring arm 110 experiencesthe maximum amount of strain at the interface with spring anchor 120. Inan embodiment, strain sensor 540 spans the interface of spring anchor120 and spring arm 110, so as to be subject to the maximum amount ofstress associated with the deflection of spring arm 110.

In an embodiment, strain sensor 540 includes a piezoelectric material. Apiezoelectric material accumulates charge in response to an appliedmechanical stress. The accumulation of charge along strained surfaces ofa piezoelectric sensor can generate a measurable voltage related to theamount of strain. As such, as spring arm 110 deflects into space 112,the voltage between the upper and lower surface of the strain sensorincreases as the strain at the interface of spring anchor 120 and springarm 110 increases, enabling calculation of the amount of deflection ofspring arm 110. Piezoelectric materials include, for example,crystalline materials such as quartz and ceramic materials such as leadzirconate titanate (PZT).

In another embodiment, strain sensor 540 is formed from a piezoresistivematerial. The electrical resistivity of a piezoresistive materialchanges in response to an applied mechanical stress. As such, strainsensor 540 may be subject to an electrical current, so that when springarm 110 deflects into space 112, the electrical resistivity of strainsensor 540 increases as the strain at the interface of spring anchor 120and spring arm 110 increases, causing a measurable increase in thevoltage across the sensor. The amount of deflection can be calculatedfrom the changes in voltage. Piezoresistive materials include, forexample, polycrystalline silicon, amorphous silicon, monocrystallinesilicon, or germanium.

In an embodiment, spring arm 110 oscillates at a resonant frequency dueto contact with a bare substrate or a micro device, as discussed above.In an embodiment, the oscillation results in a correspondinglyoscillating amount of strain at the interface of spring anchor 120 andspring arm 110, which can be detected by strain sensor 540. Thedifferences in oscillating strain for a spring arm 110 that hascontacted a bare substrate versus a spring arm 110 that has picked up amicro device enable the use of a piezoelectric or piezoresistive strainsensor 540 to determine if a transfer head has successfully picked up amicro device during a pickup operation.

Referring to FIG. 5B, a sensor electrode 542 is formed on substrate 102,according to an embodiment of the invention. In an embodiment, sensorelectrode 542 is formed within space 112, in alignment with mesastructure 104. In an embodiment, a layer of dielectric material 544isolates sensor electrode 542 from semiconductor substrate 102. Inanother embodiment, sensor electrode 542 is formed directly on basesubstrate 102. In an embodiment, dielectric layer 114 covers opposingelectrode 542.

In an embodiment, sensor electrode 542 and bottom electrode 108 functiontogether as a capacitive sensor. The capacitance between two parallelconductors increases as the distance between the conductors decreases.In an embodiment, a voltage is applied across sensor electrode 542 andbottom electrode 108. As the spring arm 110 is depressed within space112 toward base substrate 102, the distance between electrodes 542 and108 decreases, causing the capacitance between them to increase. In thismanner, the amount of deflection of spring arm 110 can be calculatedfrom changes in the capacitance between electrodes 542 and 108 acrossdielectric layer 114 and space 112. Dielectric 114 prevents shortingbetween the electrodes in the case where spring arm 110 is fullydepressed within space 112. Sensor electrode 542 may be formed from anysuitable conductive material, such as those discussed above with respectto electrodes 106 and 108.

In another application, sensor electrode 542 may be used to measure theresonant frequency of spring arm 110. As discussed above, in anembodiment, spring arm 110 oscillates at a resonant frequency determinedin part by the weight of elements forming spring arm 110. Theoscillation may result in a corresponding oscillating capacitancebetween sensor electrode 542 and bottom electrode 108. After a microdevice has been picked up by the transfer head, the additional weight ofthe micro device may change the resonant frequency of spring arm 110,resulting in changes in the oscillating capacitance as measured byelectrodes 542 and 108. In this manner, electrodes 542 and 108 may beused to determine if a transfer head has successfully picked up a microdevice during a pickup operation. Additionally, it may be determinedwhether to apply a cleaning operation to the transfer head array ormicro device array prior to reattempting a pick up operation.

FIGS. 6A-6I illustrate a method for forming a micro device transfer headusing a negative lift-off technique, according to an embodiment of theinvention. In an embodiment, a substrate 602 having a dielectric layer601 formed thereon is provided, as shown in FIG. 6A. Substrate 602 anddielectric layer 601 have the characteristics discussed above withrespect to substrate 102 and 101, respectively. In an embodiment, basesubstrate 602 is silicon and dielectric layer 601 is silicon dioxide.Dielectric layer 601 and base substrate 602 may include routing (notshown) to control the subsequently formed transfer head. The thicknessof dielectric layer 601 will correspond to the thickness of thesubsequently formed space between the spring arm and the base substrate,and as such is chosen to enable the desired degree of deflection of thetransfer head. In an embodiment, dielectric layer 601 is 0.5-2 μm thick.

Next, in FIG. 6B, first pattern 640 is formed on the surface ofdielectric layer 601 to define a portion of the bottom electrode of thetransfer head. In an embodiment, first pattern 640 is formed in theareas where electrode metal is not desired. For example, first pattern640 may be formed along the perimeter of what will be the length andwidth of the spring arm portion of the transfer head. In an embodiment,first pattern 640 is formed from photoresist.

In FIG. 6C, first electrode layer 650 is formed over the surface ofdielectric layer 601 and first pattern 640, according to an embodimentof the invention. First electrode layer 650 may be formed from any metalor conductive material suitable for forming the electrode elements ofthe spring arm of a transfer head, as discussed above with respect tomaterials for electrodes 106 and 108. First electrode layer 650 may beformed using any appropriate method, such as plating, CVD, or PVD. In anembodiment, first electrode layer 650 is formed by sputter deposition.First electrode layer 650 may be from 0.1-1 μm thick. In an embodiment,first electrode layer 650 is a 500 Å thick layer of TiW. In anembodiment, first pattern 640 may then be removed along with the portionof first electrode layer 650 formed thereon.

Referring now to FIG. 6D, a template is formed over first electrodelayer 650. In an embodiment, the template includes gap material 616 andmesa structure 604. Mesa structure 604 may be formed from the same ordifferent material than gap material 616. In one embodiment, mesastructure 604 is integrally formed with gap material 616. In anembodiment, the materials filling gap material 616 and mesa structure604 are selected to be resistant to other processes used to define thetransfer head. In another embodiment, the materials filling gap material616 and mesa structure 604 are selected to be removed during subsequentprocessing to form the transfer head.

Gap material 616 and mesa structure 604 may each be formed from avariety of materials, including photoresist, polyimide, amorphoussilicon, polysilicon, or dielectric materials, for example inorganicdielectrics such as silicon dioxide and silicon nitride, or organicdielectrics such as benzocyclobutene (BCB). For example, where gapmaterial 616 and mesa structure 604 are formed from a dielectric such assilicon dioxide or silicon nitride, a layer of dielectric may first beblanket deposited over first electrode layer 650 and then patterned byany appropriate method known in the art to form the template comprisinggap material 616 and mesa structure 604. In an embodiment, anisotropicetching techniques can be utilized to form tapered sidewalls for mesastructure 604. In another embodiment, gap material 616 and mesastructure 604 are defined using a photoresist material that ishard-baked to resist subsequent processing to form the transfer head. Inanother embodiment, gap material 616 and mesa structure 604 are formedfrom photo-defined polyimide.

The dimensions of the gap material 616 and mesa structure 604 aredetermined by the desired dimensions of the top surface of the transferhead after the formation of additional device components, such as theelectrodes and the dielectric layer, as described above with respect toFIGS. 1-2. In an embodiment, gap material 616 is 2 to 5 μm thick. In anembodiment, mesa structures 604 are 7 μm×7 μm wide and 2 μm high.

Next, in FIGS. 6E-6G, a negative patterning technique is used to formadditional components of the electrodes. In FIG. 6E, second pattern652A/652B is formed on mesa structure 604, according to an embodiment ofthe invention. In an embodiment, second pattern 652A defines the spaceseparating the two electrodes to be formed on top of mesa structure 604.In an embodiment, second pattern 652B defines what will be the end ofthe spring arm portion of the transfer head.

Referring to FIG. 6F, second electrode layer 654 is blanket depositedover the top surfaces of the template comprising gap material 616, mesastructure 604, and second pattern 652A/652B, according to an embodimentof the invention. Similarly to first electrode layer 654, secondelectrode layer 654 may be formed using a non-conformal method, such asPVD. In an embodiment, second electrode layer 654 is formed from thesame material as first electrode layer 650. Second electrode layer 654may be from 0.1 to 1 μm thick. In an embodiment, second electrode layer650 is a 500 Å thick layer of TiW.

Next, in FIG. 6G, second pattern 652A/652B is removed, along with theportions of second electrode layer 654 formed thereon, according to anembodiment of the invention. In an embodiment, removal of second pattern652A/652B defines top electrode 606 from second electrode layer 654. Abottom electrode 608 is defined by the remaining portion of firstelectrode layer 650 and a portion of the second electrode layer,according to an embodiment. In an embodiment, bottom electrode 608 runsunderneath mesa structure 604 and wraps around and over mesa structure604. In an embodiment, the removal of second pattern 652A defines space607 between top electrode 606 and bottom electrode 608. In a case wherethe template comprising gap material 616 and mesa structure 604 isformed from photoresist, the photoresist template may also be removedduring the process used to remove second pattern 652A/652B.

Referring to FIG. 6H, a portion of dielectric layer 601 is then etchedto form space 612, according to an embodiment of the invention. In anembodiment, dielectric layer 601 material is laterally etched fromunderneath bottom electrode 608 to create spring arm 610 of a desiredlength. An amount of dielectric layer 601 material may remain on thesurfaces of bottom electrode 608 and/or substrate 602 after theformation of space 612. Spring arm 610 is attached to dielectric layer601 via spring anchor 620. Dielectric layer 601 material may be removedby any appropriate method, for example, a timed wet etch process. In anembodiment, space 612 is formed by using a wet etch process that isselective to dielectric layer 601 material over bottom electrode 608material and substrate 602 material. In an embodiment, the process usedto remove dielectric material 601 to create space 612 also removes thedielectric material forming the template comprising gap material 616 andmesa structure 604, so that gap material 616 and mesa structure 604 areair-filled. In an embodiment, the process used to remove dielectricmaterial is precisely controlled to laterally etch underneath bottomelectrode 608 to form spring arm 610 without etching portions ofdielectric material that are critical to the transfer head structure. Inanother embodiment, the dielectric material forming the templatecomprising gap material 616 and mesa structure 604 has been selected sothat it is not etched by the process used to remove a portion ofdielectric layer 601.

Dielectric layer 614 is then conformally deposited over the surfaces ofspring arm 610 and underlying space 612, as shown in FIG. 6I, accordingto an embodiment of the invention. In an embodiment, dielectric layer614 conforms to the surfaces of electrodes 606/608 and the surfaces ofdielectric layer 601 and substrate 602 exposed within space 612. In anexample embodiment where mesa structure 604 and gap material 616 areair-filled, dielectric layer 614 may also form on the surfaces of topelectrode 606 and bottom electrode 608 that are internal to the springarm 610. In an embodiment, dielectric layer 614 has a suitable thicknessand dielectric constant for achieving the required grip pressure of themicro device transfer head, and sufficient dielectric strength to notbreak down at the operating voltage. Dielectric layer 614 may be asingle layer or multiple layers. Suitable dielectric materials mayinclude, but are not limited to, Al₂O₃, Ta₂O₅, HfO₂, and SiO₂ asdescribed above with respect to dielectric layer 114. In an embodiment,dielectric layer 614 is from 0.5 to 1 μm thick. In an embodiment,dielectric layer 614 is a 0.5 μm thick layer of Al₂O₃. In an embodiment,dielectric layer 614 is deposited by atomic layer deposition (ALD).

FIGS. 7A-7G illustrate a method for forming a micro device transferhead, according to an embodiment of the invention. In an embodiment, asubstrate 702 is provided, having dielectric layer 701 and firstelectrode layer 750 formed thereon, as shown in FIG. 7A. In anembodiment, substrate 702, dielectric layer 701, and first electrodelayer 750 may each have the characteristics discussed above with respectto substrate 602, dielectric layer 601, and first electrode layer 650,respectively. In an embodiment, base substrate 702 is silicon,dielectric layer 701 is silicon dioxide, and first electrode layer 750is TiW. Dielectric layer 701 may be from 0.5-2 μm thick. First electrodelayer 750 may be from 100 to 1000 nm thick.

Referring to FIG. 7B, a template comprising gap material 716 and mesastructure 704 is formed over first electrode layer 750. The materialsand dimensions of gap material 716 and mesa structure 704 may be as forgap material 116/616 and mesa structure 104/604, respectively, discussedabove. The materials used to form gap material 716 and mesa structure704 may be selected to either be resistant to subsequent fabricationprocesses or, alternatively, to be removed during subsequent processes.In an embodiment where gap material 716 and mesa structure 704 areformed from the same material as dielectric layer 701, first electrodelayer 750 protects the surface of dielectric layer 701 during theetching processes that define gap material 716 and mesa structure 704.

Next, in FIG. 7C, second electrode layer 754 is formed, according to anembodiment of the invention. In an embodiment, second electrode layer754 is blanket deposited over the exposed top surfaces of gap material716, mesa structure 704, and dielectric layer 701 according to anembodiment of the invention. Second electrode layer 754 may be formed byany appropriate method, such as CVD or PVD. Second electrode layer 754is formed from any conductive material suitable for the formation ofelectrodes, as discussed above with respect to second electrode layer654.

Next, first electrode layer 750 and second electrode layer 754 arepatterned to form top and bottom electrodes, according to an embodimentof the invention. In FIG. 7D, pattern 756A/756B is formed over secondelectrode layer 754, according to an embodiment. In an embodiment,pattern 756A covers the portion of second electrode layer 754 that willform the top electrode. In an embodiment, pattern 756B covers theportion of second electrode layer 754 that will form a portion of thebottom electrode. Pattern 756A/756B may be formed from any suitablematerial, such as a photoresist or hardmask material.

The portions of second electrode layer 754 and first electrode layer 750that are not protected by pattern 756A/756B are then etched to definetop electrode 706 and bottom electrode 708, according to an embodiment.Second electrode layer 754 and first electrode layer 750 may be etchedby any appropriate method. In an embodiment, a wet etch having aselectivity of at least 10:1 for the material forming electrode layers750/754 over the material forming the mesa structure 704 is used. In anembodiment, the wet etch used to pattern electrode layers 750/754 has aselectivity of at least 10:1 for the material forming electrode layers750/754 over the material forming dielectric layer 701.

Pattern 756A/756B may then be removed by an appropriate method, as shownin FIG. 7E. In an embodiment, the process used to remove pattern756A/756B also removes the material filling gap material 716 and mesastructure 704. For example, where the template including gap material716 and mesa structure 704 is formed from photoresist, the process thatremoves pattern 756A/756B may also remove the photoresist filling gapmaterial 716 and mesa structure 704. In another embodiment, the materialfilling gap material 716 and mesa structure 704 is not removed by theprocess used to remove pattern 756A/756B.

Next, as shown in FIG. 7F, a portion of dielectric layer 701 is removedfrom between bottom electrode 708 and substrate 702 to form space 712,according to an embodiment of the invention. In an embodiment,dielectric layer 701 is laterally etched underneath bottom electrode 708to form space 712 and spring arm 710, which is suspended above space 712and attached to dielectric layer 701 at spring anchor 720. Dielectriclayer 701 may be removed by any appropriate process, such as a timed wetetch. As discussed above with respect to FIG. 6H, in an embodiment,where the material forming the template comprising gap material 716 andmesa structure 704 is a dielectric material, the process used to removea portion of dielectric layer 701 may also remove all or a portion ofthe dielectric material filling the template, forming a hollow springarm 710. In an embodiment, the process used to remove dielectricmaterial is precisely controlled to laterally etch underneath bottomelectrode 708 to form spring arm 710 without etching dielectric materialthat is critical to the transfer head structure.

Referring to FIG. 7G, dielectric layer 714 is then conformally depositedover the surfaces of the spring arm 710 and underlying space 712. In anembodiment, dielectric layer 714 conforms to the surfaces of electrodes706/708 and the surfaces of dielectric layer 701 and substrate 702exposed within space 712. In an example embodiment where mesa structure704 and gap material 716 are air-filled, dielectric layer 714 may alsoform on the surfaces of top electrode 706 and bottom electrode 708 thatare internal to the spring arm 710. In an embodiment, the dielectriclayer 714 has a suitable thickness and dielectric constant for achievingthe required grip pressure of the micro device transfer head, andsufficient dielectric strength to not break down at the operatingvoltage. The dielectric layer 714 may be a single layer or multiplelayers. Suitable dielectric materials may include, but are not limitedto, Al₂O₃, Ta₂O₅, HfO₂, and SiO₂ as described above with respect todielectric layer 114. In an embodiment, dielectric layer 714 is from 0.5to 1 μm thick. In an embodiment, dielectric layer 714 is a 0.5 μm thicklayer of Al₂O₃. In an embodiment, dielectric layer 714 is deposited byatomic layer deposition (ALD).

In utilizing the various aspects of this invention, it would becomeapparent to one skilled in the art that combinations or variations ofthe above embodiments are possible for forming a micro device transferhead and head array, and for transferring a micro device and microdevice array. Although the present invention has been described inlanguage specific to structural features and/or methodological acts, itis to be understood that the invention defined in the appended claims isnot necessarily limited to the specific features or acts described. Thespecific features and acts disclosed are instead to be understood asparticularly graceful implementations of the claimed invention usefulfor illustrating the present invention.

What is claimed is:
 1. A micro device transfer head comprising: a basesubstrate; a spring anchor coupled to the base substrate; a spring armcoupled to the spring anchor, the spring arm including: a mesa structuresuspended over the base substrate; a top electrode extending laterallyfrom the spring anchor and over the mesa structure; and a bottomelectrode extending laterally from the spring anchor underneath the mesastructure and wrapping around and over the mesa structure, wherein thetop electrode and the bottom electrode are electrically isolated fromeach other; and a dielectric layer covering the top electrode and thebottom electrode over the mesa structure.
 2. The micro device transferhead of claim 1, wherein the spring arm further comprises a templatedefined by the top electrode and the bottom electrode, wherein thetemplate comprises the mesa structure.
 3. The micro device transfer headof claim 2, wherein the template is filled with a dielectric material.4. The micro device transfer head of claim 2, wherein the template isfilled with air.
 5. The micro device transfer head of claim 1, whereinthe top electrode and the bottom electrode are metal.
 6. The microdevice transfer head of claim 1, wherein the spring arm is deflectableinto an underlying space.
 7. The micro device transfer head of claim 6,wherein the spring arm is coupled to the base substrate via a layer ofdielectric material, and wherein the underlying space is a cavity in thedielectric layer.
 8. The micro device transfer head of claim 6, furthercomprising a sensor electrode on a bottom surface of the underlyingspace, wherein the sensor electrode is vertically aligned with the mesastructure.
 9. The micro device transfer head of claim 6, wherein thedielectric layer conforms to the spring arm and the underlying space.10. The micro device transfer head of claim 9, further comprising astrain sensor formed over a portion of the dielectric layer covering aninterface of the spring anchor and the spring arm.
 11. The micro devicetransfer head of claim 1, wherein the top surface of the transfer headhas a length from 1 to 100 μm and a width from 1 to 100 μm.
 12. Themicro device transfer head of claim 1, further comprising routing in thebase substrate, wherein the routing is electrically connected to the topelectrode and the bottom electrode.
 13. The micro device transfer headof claim 12, wherein the routing comprises through vias in the basesubstrate.
 14. The micro device transfer head of claim 1, wherein thetop electrode and bottom electrode are form a bipolar electrode on a topsurface of the mesa structure.
 15. A micro device transfer head arraycomprising: a base substrate; and an array of transfer heads, eachtransfer head including: a spring anchor coupled to the base substrate;a spring arm coupled to the spring anchor, the spring arm including: amesa structure suspended over the base substrate; a top electrodeextending laterally from the spring anchor and over the mesa structure;and a bottom electrode extending laterally from the spring anchorunderneath the mesa structure and wrapping around and over the mesastructure, wherein the top electrode and the bottom electrode areelectrically isolated from each other; and a dielectric layer coveringthe top electrode and the bottom electrode over the mesa structure. 16.The micro device transfer head array of claim 15, wherein each springarm is deflectable into an underlying space.
 17. The micro devicetransfer head array of claim 16, wherein the underlying space is atrench extending underneath a plurality of transfer heads.
 18. The microdevice transfer head array of claim 15, further comprising routing inthe base substrate, wherein the routing is electrically connected to thetop electrode and bottom electrode of each transfer head.
 19. The microdevice transfer head of claim 18, wherein the routing comprises throughvias in the base substrate.
 20. The micro device transfer head array ofclaim 15, further comprising a conductive ground plane formed over thedielectric layer and surrounding each of the transfer heads.
 21. Themicro device transfer head of claim 15, wherein the top electrode andbottom electrode of each transfer head form a bipolar electrode on a topsurface of the mesa structure.